This is a double blind, controlled, cross-over comparison of adding sitagliptin (or placebo) to pre-existing metformin+liraglutide therapy. Patients with type 2 diabetes mellitus (T2DM) on pre-existing treatment with metformin (≥ 1500 mg/d) monotherapy or metformin plus liraglutide (1.2 mg/d) will be studied. Patients on metformin monotherapy will, after screening and randomization, enter a run-in period of 2 weeks with the additional treatment of liraglutide (0.6 mg/d for 1 week followed by 1.2 mg/d for another week). At the end of this 2 weeks therapy, a mixed meal challenge will take place, with the assessment of glucose and hormone responses (insulin, C-peptide, glucagon, GLP-1 [glucagon-like peptide-1], GIP (gastric inhibitory peptide) and gastric emptying as measured by 13C (carbon 13)-octanoate breath tests. Prior to the meal tests, liraglutide will be administered at a dose of 1.2 mg per injection, which is the recommended dose for treatment. Sitagliptin will be used at a dose of 100 mg, which is recommended for clinical use.